1. Field of the Invention
The present application relates to integrated circuit (IC) metrology and more particularly to selecting a hypothetical profile for optical metrology.
2. Related Art
With the current drive towards smaller geometries of IC features, feature measurement is increasingly difficult as the size of the features becomes smaller. However, knowledge of the dimensions of gratings or periodic structures is essential in order to determine if the dimensions of the features are within the acceptable ranges and if, for example, a particular fabrication process causes the sidewalls of the features to be tapered, vertical, T-topped, undercut, or have footings.
Traditionally, a sample was cleaved and examined with a scanning electron microscope (SEM) or similar device. The cross-section SEM method is typically slow, expensive, and destructive, whereas the critical dimension (CD) SEM method only provides one measurement number seen from the top of the feature. Spectroscopic reflectometry and ellipsometry are used to beam light on the structure and measure the reflected beam. One application uses an empirical approach, where the spectra of reflected light are measured for a known width of features in a structure. This process is time consuming and expensive, even for a limited library of profiles of structure dimensions and the associated spectrum data of reflected/diffraction light. Furthermore, there is also a need for faster determination of CD's for in-line integrated metrology applications. Additionally, as the resolution of the library increases, the size of the library increases, while the time to create and use the library increases exponentially.
One technique for creating a library of diffraction signals and associated profiles of the integrated circuit structure involves the use of a hypothetical profile of the structure to calculate a simulated diffraction signal. The length of time needed to create a library using this approach varies depending on the hypothetical profile used and the number of parameters used to represent the hypothetical profile. Typically, the more complicated the hypothetical profile and the more parameters used, the more time and/or computing resources are needed.